HE used to be the next president of the USA. Now he fends off talk of a return to politics because he has to save the planet.

After that, who knows? When I ask Al Gore whether he will run for president again, he cocks his head, gives a slight smile and whispers: "That's a different challenge."

He was America's 45th vice-president, serving two terms under Bill Clinton. Then he ran for the top job and lost to George W Bush in the closest race in election history.

Many say he was robbed by questionable vote-counting in Florida (governed by Bush's brother Jeb) and a hostile Supreme Court. But, if there is disappointment, Gore does not show it. Instead, he describes himself as "a recovering politician", whose mission these days is leading the fight against climate change.

In July - 7/7/07 to be precise - two billion viewers will watch Live Earth, a global music-fest designed to trigger a worldwide movement to combat the environmental mess we have created.

More than 100 bands are to perform in all seven continents, with Wembley's new stadium the venue for Europe's main gig.

The 24-hour gala will be transmitted via TV, radio, the internet and other media - with a million-plus live concert-goers.

"It is an event which I hope will touch all of us," says Gore. "It focuses on an issue which affects everyone. And each one of us must be part of the solution.

"We hope that Live Earth will give people the tools they need to help solve the climate crisis. But, ultimately, corporations and governments must become global leaders - and take decisive action to stop global warming."

Proceeds from the concerts will create a foundation led by the Alliance For Climate Protection, currently chaired by Gore.

We are sitting in a suite in London's Dorchester Hotel. He arrived only an hour earlier on the "red eye" flight from New York. There's no trace of fatigue. He's used to flying, though these days he balances that jet travel through carbon offsetting.

"I spent eight years on Air Force Two. Sure, it was more comfortable and, these days, I have to take my shoes off and have them security-checked to get on a plane. But that's no hardship."

THEN Gore adds that he was recently driving his wife Tipper from their home in Nashville to the family farm in Tennessee.

"And I know that doesn't sound like a big deal but I looked in the rear-view mirror and there was no motorcade... ever heard of phantom limb pain?"

You know he's been cracking that joke for the past couple of years. But it's still funny - a well-rehearsed warm-up routine, distancing himself from the political past and into his "carbon zero" campaign.

It is based on An Inconvenient Truth, the double Oscar-winning documentary of the one-man show he has been touring round the world for the past three years.

That film highlighted the perils of global warming, foreseeing a bleak future tempered by the belief that we can still prevent it. "What is needed is the will," he says. "And I believe people and political systems can and will respond. I've seen it so many times before - lots of occasions when it seemed that nothing was happening, nothing changing. Then, suddenly, we come to the tipping point and dramatic change does take place."

Last week, the United Nations climate change panel issued a grim report on the effects that even a 1C rise in global temperatures could bring - heatwaves, floods, storms, fires and droughts. By 2080, it claimed, up to 3.2 billion people could face water shortages and up to 600 million may be hit by famine.

"The evidence is there," Gore agrees. "And we owe it to our children and grandchildren to act now.

"Believe me, this isn't my bandwagon. This is an issue beyond dispute and it's bad-to-catastrophic.

"We're all to blame. Don't just single out China and nations on the verge of huge growth. I've met with China's leaders and they do care. They know they are riding a tiger.

"My own country hasn't been great either. We've created terrible environmental problems. And Kyoto - Bush's failure to sign the agreement limiting greenhouse gases - was a terrible disappointment. I was crushed. But governments do change. There are signs Mr Bush has changed. Meantime, my aim is to get in at grassroots level, by telling the truth."

He pauses to take a swig of Diet Coke. He looks relaxed. Dark suit, black shirt and a pair of old, black cowboy boots. I notice he has quite big feet. A large carbon-black footprint, so to speak, stomping round the world on all those planes spreading his environmental credo.

He laughs and says that the Live Earth gig has been closely monitored to prevent unsuitable sponsorship.

He adds that the shows themselves will be powered from renewable sources. Waste will be recycled, air travel will be offset through carbon credits, ground travel will use hybrid or clean fuel. Everything nailed down...

Then I try to ask him about his Nashville mansion, recently shown to be hugely energy inefficient.

But that's when his publicity officer calls a halt, claiming we're out of time.

Gore wants to answer. But there's no chance. The mission to save the planet is too far behind schedule.